Keep in mind that most modern hardware works well with Linux. In fact, Ubuntu or Fedora has never failed to detect any of my PC hardware. Even the proprietary graphics driver and the wireless drivers installed with a breeze. However, you may not be as lucky as I have been. Therefore it is recommended to search for Linux compatible hardware before you plan on making a purchase. Many sites on the internet maintain a list of computer hardware which is compatible with a particular Linux variant. Ubuntu has a hardware certification program which lists Ubuntu certified or 100% compatible hardware.
At present, there are more than 90 certified Dell laptops (including models from the Inspiron and Latitude series). Besides Dell, the website lists servers from IBM and Intel, products from Toshiba, Asus, Acer, Wipro, HCL, HP, Sun and many others. Find out more from the official website.
You must also check out H-node, which maintains a list of hardware compatible with Linux. This was previously maintained by the FSF on their website but now the information has been moved to H-node.
Besides this, OpenSUSE, Mandriva and RedHat maintain lists of compatible hardware. I'm quite sure you'll find what you need.